Ischemia is the condition or state in which a tissue such as brain is subjected to hypoxia or low oxygen because of an obstruction of the arterial blood supply or inadequate blood flow. Brain ischemia can be broadly divided into two main classifications, global ischemia and focal ischemia. Ischemia is insufficient blood flow to provide adequate oxygenation. This, in turn, leads to tissue hypoxia (reduced oxygen) or anoxia (absence of oxygen). Ischemia always results in hypoxia; however, hypoxia can occur without ischemia if, for example, the oxygen content of the arterial blood decreases as occurs with anemia.The most common causes of ischemia are acute arterial thrombus formation, chronic narrowing (stenosis) of a supply artery that is often caused by atherosclerotic disease, and arterial vasospasm.   
As blood flow is reduced to an organ, oxygen extraction increases.  When the tissue is unable to extract adequate oxygen, the partial pressure of oxygen within the tissue falls (hypoxia) leading to a reduction in mitochondrial respiration and oxidative metabolism. If the hypoxic state is prolonged, cellular death may occur.Ischemia can also lead to ischemic pain. For example, myocardial ischemia can cause chest pain (angina); leg ischemia can result in ischemic pain at rest and/or during physical activity.